Humoral immune responses play a major role in the resistance to viral infections and passive immunization is effective in the prophylaxis and therapy of many viral diseases. The present proposal is based on an antibody molecular cloning technology developed in our Institution, in which antibody Fab sequences are cloned into a filamentous phage vector to produce phage display libraries. Positive clones are then selected by affinity sorting using immobilized antigen. With this technology, we have recently generated recombinant monoclonal antibody Fab fragments against different pathogenic viruses including HIV and HSV. Some of these antibodies have proven effective in in vitro neutralization paradigms. The proposed research will generate and characterize specific recombinant antibodies against SIV from SIV-infected macaques with special emphasis on HIV-SIV crossreacting antibodies. We will also isolate such antibodies from Fab libraries established from HIV-1- infected individuals. This approach will generate large panels of recombinant monoclonal antibodies which will allow features such as antibody synergy and antibody-mediated enhancement to be assessed. Lastly, the proposed research will lay the foundations for the characterization of the role of passive immunization in the prevention and treatment of lentivirus infection in vivo in the macaque model of AIDS. Selected monoclonal antibodies should prove more effective than polyclonal antisera since the latter may only contain beneficial antibodies at low titer and may also contain detrimental and infection enhancing antibodies. This research will contribute to the understanding of humoral immune response against retroviruses and provide opportunities to explore the mechanisms underlying antibody protective actions.